[nat64] run nat64 ci tests as an independent job (#7885)

This commit changes the NAT64 CI tests to be ran as an independent
job. A new /nat64 folder is added and it would make it easier to
configure new NAT64 tests in the future.

script/test

@@ -324,7 +324,7 @@ do_build_otbr_docker()
             --build-arg BACKBONE_ROUTER=1 \
             --build-arg REFERENCE_DEVICE=1 \
             --build-arg OT_BACKBONE_CI=1 \
-            --build-arg NAT64=0 \
+            --build-arg NAT64="${NAT64:-0}" \
             --build-arg REST_API=0 \
             --build-arg WEB_GUI=0 \
             --build-arg MDNS="${OTBR_MDNS:-mDNSResponder}" \